What it means
だけでなく means “not only ... but also ...”. It adds a second item, quality, or action while emphasizing that the first one is also true. It can attach to nouns, adjectives, and plain-form verbs, and the second part commonly uses も, まで, or a similar additive expression. The whole sentence can still be made polite by changing the sentence ending, although だけでなく itself does not have a separate polite form.
Usage note
だけでなく usually introduces a second item or action, so the following phrase should make the added information clear. Do not confuse it with だけ, which means “only” and does not itself add a second item.
Example sentences
この図書館では、本だけでなく、雑誌や新聞も無料で読めます。
At this library, you can read not only books but also magazines and newspapers for free.
この電車は駅に止まるだけでなく、車内で無料の飲み物も提供します。
This train not only stops at stations but also provides free drinks inside the train.